This invention relates to an actuator for the brake shoe of a drum brake. The invention will be particularly described with reference to a drum brake of a leading-trailing configuration, having a substantially circular single shoe. It is to be understood however, that the invention is also applicable to drum brakes having different shoe configurations.
In the manufacture of brakes for motor vehicles and the like there is a need to have brake assemblies which utilise a low number of parts. Assemblies of this nature are advantageous for various reasons including a decreased chance of assembly failure, and a decrease in the time required to assemble the brake. There is also generally an associated decrease in cost of the assembly. Hydraulic brakes of this type have usually employed at least one piston/cylinder assembly which has two pistons movable in opposite directions within the cylinder, and the pistons are arranged to contact opposite ends of the brake shoe assembly. This arrangement requires separate seals for each of the two pistons thereby adding further to the number of components required for the total assembly. Single piston assemblies have been used in the past but have been considered not satisfactory because of the need for the piston/cylinder assembly to slide relative to the back plate on which the brake assembly is mounted. The sliding interfaces are a major source of problem due to the possibility of corrosion and seizure taking place.
Drum brake assemblies which include both service and park brake actuating mechanisms generally involve use of a park brake lever which is mechanically operable independently of a hydraulic service brake system. Prior arrangements of that kind have suffered from various problems. In one such prior arrangement, the park brake lever extends through the back plate for operation outside of the brake assembly in a plane which is generally parallel to the rotational axis of the brake drum. That arrangement, known as a cross-pull lever, has the disadvantage that the lever must pass through the back plate and must thus be separately sealed or booted at the location where it passes through the back plate to avoid the ingress of dirt into the brake assembly. The provision of the boot seal adds to the number of parts for the brake assembly. In another arrangement, known as an in-plane lever, the park brake lever operates in a plane generally transverse to the rotational axis of the drum. A problem with that arrangement is that the actuation points on the brake shoe for the park brake mechanism need to be separate from the actuation points for the service brake. That arrangement adds to the number of parts for the mechanism. Also, provision of separate actuation points is not practicable for some brake shoe configurations, particularly single piece shoe brakes having a channel shaped shoe body.
It is a requirement of modern brake assemblies that the mechanism is automatically adjustable to compensate for pad wear in use. It is a further requirement that the automatic adjustment mechanism is capable of operating as a result of service brake use, irrespective of whether the park brake is used. The mechanism should also be relatively simple and fail safe.